T Pyxidis (T Pyx)[1] is a recurrent nova[2] and nova remnant in the constellationPyxis. It is a binary star system and its distance is estimated at about 4,783 parsecs (15,600 light-years) from Earth. It contains a Sun-like star and a white dwarf. Because of their close proximity and the larger mass of the white dwarf, the latter draws matter from the larger, less massive star. The influx of matter on the white dwarf's surface causes periodic thermonuclear explosions to occur.

The usual apparent magnitude of this star system is 15.5, but there occurred eruptions with maximal apparent magnitude of about 7.0 in the years 1890, 1902, 1920, 1944, 1966 and 2011.[3] Evidence seems to indicate that T Pyxidis may have increased in mass despite the nova eruptions, and is now close to the Chandrasekhar limit when it might explode as a supernova.[4] When a white dwarf reaches this limit it will collapse under its own weight and cause a type 1a supernova.

Effect on Earth

Because of its relative proximity, some—in particular, Edward Sion, astronomer & astrophysicist at Villanova University, and his team therefrom—contend that a type 1a supernova could have a significant impact on Earth. The received gamma radiation would equal the total (all spectra) radiation of approximately 1,000 solar flares,[5] but the type Ia supernova would have to be closer than 1,000 parsecs (3,300 light-years) to cause significant damage to the ozone layer, and perhaps closer than 500 parsecs. The X-radiation that reaches Earth in such an event, however, would be less than the X-radiation of a single average solar flare.[5]

However, Sion's calculations were challenged by Alex Filippenko of the University of California at Berkeley who said that Sion had possibly miscalculated the damage that could be caused by a T Pyxidis supernova. He had used data for a far more deadly gamma-ray burst (GRB) occurring 1 kiloparsec from Earth, not a supernova, and T Pyxidis certainly is not expected to produce a GRB.[6] According to another expert, "[a] supernova would have to be 10 times closer [to Earth] to do the damage described."[6]Mankind survived when the Crab Nebula supernova went off at a distance of about 6,500 light-years in the year 1054. If this star were to explode as a type Ia supernova at its estimated distance of 3,300 light-years, it would have an apparent magnitude of around -9.3, about as bright as the brightest Iridium (satellite) flares.[7]

Recent data indicates his distance estimate is five times too close. Astronomers used NASA's Hubble Space Telescope to observe the light emitted during its latest outburst in April 2011. The team also used the light echo to refine estimates of the nova's distance from Earth. The new distance is 15,600 light-years (4780 pc) from Earth. Previous estimates were between 6,500 and 16,000 light-years (2000 and 4900 pc).[8]

It has been reported that T Pyx would "soon" become a supernova.[4] However, when contacted by Scientific American, it became apparent that "soon" was meant in astronomical terms: Dr. Sion said that "soon" in the press announcement meant "[a]t the accretion rate we derived, the white dwarf in T Pyxidis will reach the Chandrasekhar Limit in ten million years."[9] By that time it will have moved far enough away from the solar system to have little effect.

2011 Outburst

Mike Linnolt has detected T Pyx first outburst in nearly 45 years. He detected it on April 14, 2011 at magnitude 13.[3] According to AAVSO observers, it reached magnitude 7.5 in the visual and V bands by April 27,[10]and reached magnitude 6.8 by May 3.[11]

X-ray source

A nova remnant is made up of the material either left behind by a sudden explosive fusion eruption by classical novae, or from multiple ejections by recurrent novae. Over their short lifetimes, this novae shell shows expansion velocities are around 1000 km/s[1], whose faint nebulosity usually are illuminated by their progenitor stars via light echos as observed with the spherical shell[1] of Nova Persei 1901[2] or the energies remaining in the expanding bubbles like T Pyxidis.[3]

As most novae require a close binary system, with a white dwarf and main sequence, sub-giant; or the merging of two red dwarfs; so probably all novae remnants must be associated with binaries[4]. This theortically means these nebulae shapes might be affected by their central progenitor stars and the amount of matter ejected by novae.[1]Shapes of these novae nebulae is of much interest to modern astrophysicists.[1][4]

A nova (plural novae or novas) or classical nova (CN or plural CNe) is an astronomical event that causes the sudden appearance of a bright "new" star, that slowly fades from view over several weeks or many months. Novae should not be confused with other more energetic astronomical phenomena known as supernovae (SNe), which catastrophically destroys massive stars or white dwarfs.

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Recurrent novae (RNe) are objects that have been seen to experience multiple nova eruptions. There are some ten known galactic recurrent novae.[16] The recurrent nova typically brightens by about 8.6 magnitude, whereas a classic nova brightens by more than 12 magnitude.[16]

The recurrent nova T Pyxidis, which had its last outburst in December 1966 and has been very overdue for its next, finally began a new flareup on April 14th. By the next day it had brightened to about magnitude 8.4. In 1966–67 it reached 6.5 within about a month, and now it has done so again.

Here's a blink animation showing before and after, courtesy Ernesto Guido and Giovanni Sostero in Italy. South is up.

The star is in the dim constellation Pyxis east of Puppis and Canis Major, sinling out of sight for northern observers in May. (Here are finder and comparison-star charts from Sky & Telescope. For larger-scale comparison-star charts, you can use the chart-making site of the American Association of Variable Star Observers).

As a white dwarf accreting mass from a binary partner, T Pyx is a prime candidate to explode completely as a Type Ia supernova within the next 10 million years. Last year, careless reporting led to a flurry of media buzz that if it blows up soon it could endanger Earth. Not so. At its distance of about 3,000 light-years, T Pyx would shine as brightly as magnitude –9 if it went Type Ia (as bright as a thick crescent Moon), but its radiation wouldn't harm us. The current Wikipedia article on T Pyx explains this story well.

In its past known outbursts (1890, 1902, 1920, 1944, and 1966), T Pyx took about 20 days to reach 7th magnitude or so and remained 8th mag or brighter for about two months.

Elaborating on this, Bradley Schaefer writes: "Judging from the 1967 eruption light curve, the current eruption light curve will... slowly rise to a peak near V=6.4 around 20 May, slowly fade to V=10 by middle August, then have a sudden drop by two magnitudes over the next 20 days (with drop being invisible due to the Sun). The 1967 eruption did show fast intra-night variations, but the old data does not have the time resolution to tell what is going on."

UPDATE May 13: As T Pyx disappears to the twilight horizon for Northern Hemisphere observers, it has brightened to magnitude 6.5 as of last night according to various observers reporting to the AAVSO. Here are recent observations and a light curve for the most recent 50 days.

Pyxiswas formed by
La Caille from stars in the Mast of Argo, and so associated with the Ship,
although there, of course, it is an anachronism.

Baily
reannexed it to Argo, since four of its members had been placed by Ptolemy
where La Caille found them, so that for a time it fell into disuse; but
Gould inserted it in hisUranometria
Argentinaof 1879, with sixty-six stars from 3.8 to
7th magnitudes.

German astronomer Johann Bode defined the constellation Lochium Funis, the Log and Line—a nautical device once used for measuring speed and distance travelled at sea—around Pyxis in his 1801 star atlas, but the depiction did not survive.[6] In 1844 John Herschel attempted to resurrect the classical configuration of Argo Navis by renaming it Malus the Mast, a suggestion followed by Francis Baily, but Benjamin Gould restored Lacaille's nomenclature.[4]

Features

Stars

Lacaille gave Bayer designations to ten stars now named Alpha to Lambda Pyxidis, skipping the Greek letters iota and kappa. Although a nautical element, the constellation was not an integral part of the old Argo Navis and hence did not share in the original Bayer designations of that constellation, which were split between Carina, Vela and Puppis.[4] Pyxis is a faint constellation, its three brightest stars—Alpha, Beta and Gamma Pyxidis—form a rough line.[11] Overall, there are 41 stars within the constellation's borders with apparent magnitudes brighter than or equal to 6.5.[c][7]

With an apparent magnitude of 3.68, Alpha Pyxidis is the brightest star in the constellation.[13] Located 880 ± 30 light-years distant from Earth,[14] it is a blue-white giant star of spectral type B1.5III that is around 22,000 times as luminous as the Sun and has 9.4 ± 0.7 times its diameter. It began life with a mass 12.1 ± 0.6 times that of the Sun, almost 15 million years ago.[15] Its light is dimmed by 30% due to interstellar dust, so would have a brighter magnitude of 3.31 if not for this.[13] The second brightest star at magnitude 3.97 is Beta Pyxidis, a yellow bright giant or supergiant of spectral type G7Ib-II that is around 435 times as luminous as the Sun,[16] lying 420 ± 10 light-years distant away from Earth.[14] It has a companion star of magnitude 12.5 separated by 9 arcseconds.[17] Gamma Pyxidis is a star of magnitude 4.02 that lies 207 ± 2 light-years distant.[14] It is an orange giant of spectral type K3III that has cooled and swollen to 3.7 times the diameter of the Sun after exhausting its core hydrogen.[18]

Kappa Pyxidis was catalogued but not given a Bayer designation by Lacaille, however Gould felt the star was bright enough to warrant a letter.[4] Kappa has a magnitude of 4.62 and is 560 ± 50 light-years distant.[14]An orange giant of spectral type K4/K5III,[19] Kappa has a luminosity approximately 965 times that of the Sun.[16] It is separated by 2.1 arcseconds from a magnitude 10 star.[20]Theta Pyxidis is a red giant of spectral type M1III and semi-regular variable with two measured periods of 13 and 98.3 days, and an average magnitude of 4.71,[21] and is 500 ± 30 light-years distant from Earth.[14] It has expanded to approximately 54 times the diameter of the Sun.[18]

Located around 4 degrees northeast of Alpha is T Pyxidis,[22] a binary star system composed of a white dwarf with around 0.8 times the Sun's mass and a red dwarf that orbit each other every 1.8 hours. This system is located around 15,500 light-years away from Earth.[23] A recurrent nova, it has brightened to the 7th magnitude in the years 1890, 1902, 1920, 1944, 1966 and 2011 from a baseline of around 14th magnitude. These outbursts are thought to be due to the white dwarf accreting material from its companion and ejecting periodically.[24]

TY Pyxidis is an eclipsing binary star whose apparent magnitude ranges from 6.85 to 7.5 over 3.2 days.[25] The two components are both of spectral type G5IV with a diameter 2.2 times,[26] and mass 1.2 times that of the Sun, and revolve around each other every 3.2 days.[27] The system is classified as a RS Canum Venaticorum variable, a binary system with prominent starspot activity,[25] and lies 184 ± 5 light-years away.[14] The system emits X-rays, and analysing the emission curve over time led researchers to conclude that there was a loop of material arcing between the two stars.[28]RZ Pyxidis is another eclipsing binary system, made up of two young stars less than 200,000 years old. Both are hot blue-white stars of spectral type B7V and are around 2.5 times the size of the Sun. One is around five times as luminous as the Sun and the other around four times as luminous.[29] The system is classified as a Beta Lyrae variable, the apparent magnitude varying from 8.83 to 9.72 over 0.66 days.[30]XX Pyxidis is one of the more-studied members of a class of stars known as Delta Scuti variables[31]—short period (six hours at most) pulsating stars that have been used as standard candles and as subjects to study astroseismology.[32] Astronomers made more sense of its pulsations when it became clear that it is also a binary star system. The main star is a white main sequence star of spectral type A4V that is around 1.85 ± 0.05 times as massive as the Sun. Its companion is most likely a red dwarf of spectral type M3V, around 0.3 times as massive as the Sun. The two are very close—possibly only 3 times the diameter of the Sun between them—and orbit each other every 1.15 days. The brighter star is deformed into an egg shape.[31]

AK Pyxidis is a red giant of spectral type M5III and semi-regular variable that varies between magnitudes 6.09 and 6.51.[33] Its pulsations take place over multiple periods simultaneously of 55.5, 57.9, 86.7, 162.9 and 232.6 days.[21]UZ Pyxidis is another semi-regular variable red giant, this time a carbon star, that is around 3560 times as luminous as the Sun with a surface temperature of 3482 K, located some 2116 light-years away from Earth.[16] It varies between magnitudes 6.99 and 7.83 over 159 days.[34]VY Pyxidis is a BL Herculis variable (type II Cepheid), ranging between apparent magnitudes 7.13 and 7.40 over a period of 1.24 days.[35] Located around 650 light-years distant, it shines with a luminosity approximately 45 times that of the Sun.[16]

The closest star to Earth in the constellation is Gliese 318, a white dwarf of spectral class DA5 and magnitude 11.85.[36] Its distance has been calculated to be 26 light-years,[37] or 28.7 ± 0.5 light-years distant from Earth. It has around 45% of the Sun's mass, yet only 0.15% of its luminosity.[38]WISEPC J083641.12-185947.2 is a brown dwarf of spectral type T8p located around 72 light-years from Earth. Discovered by infrared astronomy in 2011, it has a magnitude of 18.79[39]

Planetary systems

Pyxis is home to three stars with confirmed planetary systems—all discovered by doppler spectroscopy. A hot Jupiter, HD 73256 b, that orbits HD 73256 every 2.55 days, was discovered using the CORALIE spectrograph in 2003. The host star is a yellow star of spectral type G9V that has 69% of our Sun's luminosity, 89% of its diameter and 105% of its mass. Around 119 light-years away, it shines with an apparent magnitude of 8.08 and is around a billion years old.[40]HD 73267 b was discovered with the High Accuracy Radial Velocity Planet Searcher (HARPS) in 2008. It orbits HD 73267 every 1260 days, a 7 billion year-old star of spectral type G5V that is around 89% as massive as the Sun.[41] A red dwarf of spectral type M2.5V that has around 42% the Sun's mass, Gliese 317 is orbited by two gas giant planets. Around 50 light-years distant from Earth, it is a good candidate for future searches for more terrestrial rocky planets.[42]

Discovered in 1995,[46] the Pyxis globular cluster is a 13.3 ± 1.3 billion year-old globular cluster situated around 130,000 light-years distant from Earth and around 133,000 light-years distant from the centre of the Milky Way—a region not previously thought to contain globular clusters.[47] Located in the galactic halo, it was noted to lie on the same plane as the Large Magellanic Cloud and the possibility has been raised that it might be an escaped object from that galaxy.[46]

NGC 2613 is a spiral galaxy of magnitude 10.5 which appears spindle-shaped as it is almost edge-on to observers on Earth.[48]Henize 2-10 is a dwarf galaxy which lies some 30 million light-years away. It is notable for having a black hole around a million solar masses at its centre. Known as a starburst galaxy due to very high rates of star formation, it has a bluish colour due to the huge numbers of young stars within it.[49]

The unique planetary nebula NGC 2818 is nested inside the open star cluster NGC 2818A. Both the cluster and the nebula reside over 10,000 light-years away, in the southern constellation Pyxis (the Compass).

NGC 2818 is one of very few planetary nebulae in our galaxy located within an open cluster. Open clusters, in general, are loosely bound and they disperse over hundreds of millions of years. Stars that form planetary nebulae typically live for billions of years. Hence, it is rare that an open cluster survives long enough for one of its members to form a planetary nebula. This open cluster is particularly ancient, estimated to be nearly one billion years old.

Planetary nebulae fade away gradually over tens of thousands of years. The hot, remnant stellar core of NGC 2818 will eventually cool off for billions of years as a white dwarf. Our own sun will undergo a similar process, but not for another 5 billion years or so.

This Hubble image was taken in November 2008 with the Wide Field Planetary Camera 2. The colors in the image represent a range of emissions coming from the clouds of the nebula: red represents nitrogen, green represents hydrogen, and blue represents oxygen.

The Pyxis globular cluster is a globular cluster in the constellationPyxis. It lies around 130,000 light-years distant from earth and around 133,000 light-years distant from the centre of the Milky Way—a distance not previously thought to contain globular clusters. It is around 13.3 ± 1.3 billion years old.[1] Discovered in 1995 by astronomer Rainer Wienberger while he was looking for planetary nebulae, it is in the Galactic halo. Irwin and colleagues noted that it appears to lie on the same plane as the Large Magellanic Cloud and raised the possibility that it might be an escaped object from that galaxy.[2]

A globular cluster is a spherical collection of stars that orbits a galactic core as a satellite. Globular clusters are very tightly bound by gravity, which gives them their spherical shapes and relatively high stellar densities toward their centers. The name of this category of star cluster is derived from the Latinglobulus—a small sphere. A globular cluster is sometimes known more simply as a globular.

Globular clusters are found in the halo of a galaxy and contain considerably more stars and are much older than the less dense open clusters, which are found in the disk of a galaxy. Globular clusters are fairly common; there are about 150[2] to 158[3] currently known globular clusters in the Milky Way, with perhaps 10 to 20 more still undiscovered.[4] These globular clusters orbit the Galaxy at radii of 40 kiloparsecs (130,000 light-years) or more.[5] Larger galaxies can have more: Andromeda Galaxy, for instance, may have as many as 500.[6] Some giant elliptical galaxies (particularly those at the centers of galaxy clusters) such as M87,[7]have as many as 13,000 globular clusters.

Every galaxy of sufficient mass in the Local Group has an associated group of globular clusters, and almost every large galaxy surveyed has been found to possess a system of globular clusters.[8] The Sagittarius Dwarf galaxy and the disputed Canis Major Dwarf galaxy appear to be in the process of donating their associated globular clusters (such as Palomar 12) to the Milky Way.[9] This demonstrates how many of this galaxy's globular clusters might have been acquired in the past.

Although it appears that globular clusters contain some of the first stars to be produced in the galaxy, their origins and their role in galactic evolution are still unclear. It does appear clear that globular clusters are significantly different from dwarf elliptical galaxies and were formed as part of the star formation of the parent galaxy rather than as a separate galaxy.[10]

The distinction between the halo and the main body of the galaxy is clearest in spiral galaxies, where the spherical shape of the halo contrasts with the flat disc. In an elliptical galaxy, there is no sharp transition between the body of the galaxy and the halo.

Sítio das Estrelas, parada de um caminho a Caminho do Céu

Stonehenge no Sítio das Estrelas

Scientists Create Most Detailed Map of the Milky Way

Scientists have created a detailed map of the Milky Way using two of the world’s largest fully steerable radio telescopes in Germany and Australia. The research looked at neutral atomic hydrogen—the most abundant element in space and the main component of stars and galaxies—across the whole sky in a survey known as HI4PI. The project required more than a million individual observations and about ten billion individual data points.